[eng] In terms of global warming, early peak forecasts in oil and natural gas seem reasonably good news because most emissions arise from fossil fuel burning. However, this can be misleading if coal resources are as enormous as some estimations report because a switch between low carbon content fossil fuels (oil and natural gas) and high carbon content (coal) can be envisaged. Following this hypothesis we develop peak oil and natural gas scenarios where coal supplies what is needed for levelling off fossil fuel energy as soon as oil and natural gas reach their joint peak. We estimate the implications in terms of greenhouse gas emissions and we compare them with the most pessimistic IPCC scenario. Our conclusion is, despite this IPCC scenario probably being unrealistic, the limitation of fossil fuel resources is not quite sufficient to avoid very dangerous emission future evolutions. CO2 concentrations well above 450 ppm at the end of the century are derived and, in addition, no sign of stabilization is observed.